A simplest control system of a legged mobile robot, in particular a biped robot comprises a desired motion pattern generator and a joint drive controller. The desired motion pattern generator generates at least a desired motion pattern. Normally, the desired motion pattern is generated in such a manner that a ZMP trajectory, obtained therefrom by conducting a dynamics calculation thereon, i.e., by solving the Euler-Newton equation, tracks a predetermined desired trajectory. The joint drive controller controls to drive the robot joints such that the joint displacement tracks the joint displacement command of the respective joints generated by the generator.
Here, the ZMP (Zero Moment Point) is used in this specification to indicate a floor point at which the moment components, except for the vertical component, of the resultant force (of the inertial force and the gravity generated by the robot motion) are both zero.
In such a control system, if the generator anticipates the floor flat and generates a gait suitable therefor, but the floor has, in fact, a slant as illustrated in FIG. 41. If the robot foreleg foot lands on the unexpected slant at the beginning of the two-leg supporting period, the foot generates an excessive floor reaction force greater than that anticipated, causing the robot to tilt. In order to solve this problem, the applicant proposed, in Japanese Laid-Open Patent Application Hei 5 (1993)-305,586, a control system for a biped robot of this kind.
In the proposed system, the robot body inclination is detected to determine a moment-restoring-demand value, while the moment component of the actual total floor reaction force about a desired total floor reaction force central point (the central point of the total floor reaction force; a desired ZMP) is detected, and the robot feet are controlled to move up and down and rotate such that the detected moment component of the actual total floor reaction force becomes equal to the moment-restoring-demand value. The moment component of the actual total floor reaction force is a moment which the resultant force of all of the foot floor reaction forces of the feet generates about the desired total floor reaction force central point (i.e., the desired ZMP).
Taking as an example the case illustrated in FIG. 41 in which the floor has the unexpected slant, the control system proposed earlier by the applicant will be explained. (The control of the system will be hereinafter referred to as "two-leg-compliance control".) For ease of understanding, each foot is assigned with a reference numeral, as illustrated in the figure. It is assumed here that, although the gait generator anticipates the floor flat and generates a gait suitable therefor, but the floor has, in fact, the slant as illustrated in FIG. 41, and hence, since the robot foreleg foot (1st foot) lands on the unexpected slant at the beginning of the two-leg supporting period, the foot generates an excessive floor reaction force greater than that anticipated. It is also assumed that the robot still keeps, at that instant, a desirable posture (i.e., the body inclination zero).
In the proposed control system, the actual total floor reaction force's moment about the desired total floor reaction force central point (i.e., the desired ZMP) is detected. At that situation, since the vertical component of the floor reaction force of the 1st foot is excessive, the actual total floor reaction force's moment acts in the direction to tilt the robot backward.
In the proposed control system, in order to decrease the moment to zero, a virtual floor A-A' is supposed, as illustrated in FIG. 42, and the virtual plane is supposed to be rotated by an angle .DELTA..theta. about the desired total floor reaction force central point (desired ZMP) and each foot is supposed to be on the virtual plain such that the feet are moved to the positions on the virtual floor.
With this, the vertical component of the 1st foot floor reaction force decreases, while the vertical component of the 2nd foot floor reaction force increases. As a result, the actual total floor reaction force's moment about the desired total floor reaction force central point (desired ZMP) becomes almost zero. Thus, even if the floor has an unexpected slant, this two-leg-compliance control can ensure that the robot continues walking without tipping over.
However, the proposed technique can not control the actual floor reaction force acting on each foot during the two-leg supporting period, if the floor has an unexpected local slant or bump, the robot is likely to spin or may tip over due to drastic posture change.
To be more specific, as illustrated in FIG. 43, if there exists an unexpected protrusion or step (level difference) on the floor at a position at which the robot 1st foot toe is scheduled to land in the two-leg supporting period, since the 1st foot toe is controlled to be driven downward in the two-leg supporting period, the 1st foot toe will stomp the projection, causing the vertical component of the 1st foot floor reaction force to grow drastically. As a result, this suddenly generates the actual total floor reaction force's moment about the desired total floor reaction force central point (desired ZMP). The two-leg-compliance control would sometimes be late in restoring the posture and at worst, the robot turns over.
Even if the robot did not turn over during the two-leg supporting period, when the second foot is lifted, although the desired total floor reaction force central point (desired ZMP) is set at the heel of the 1st foot, since the 1st foot heel is not on the floor, the actual total floor reaction force central point shifts to its toe. The generated actual moment of total floor reaction force about the desired total floor reaction force central point (desired ZMP) will tilt the robot backward and cause the robot to turn over.
It could be stated from the above that the two-leg-compliance control can effectively cope with an unexpected slant or undulation extending over a relatively long distance, but can not cope with an unexpected local slant or level difference existing at a position at which the robot foot will land.
Aside from the aforesaid two-leg-compliance control system, the applicant proposed, in Japanese Laid-Open Patent Application No. Hei 5 (1993)-305,584, etc., another control system which has a foot-landing-impact-absorber made of a material such as rubber with a springy property. In this control system, actual foot floor reaction force's moment acting about the ankle of each robot foot is detected and an ankle compliance control to rotate the foot ankle such that the detected moment becomes zero, is effected.
In order to solve the problem mentioned above, it is therefore possible to combine this control disclosed in Japanese Laid-Open Patent Application No. Hei 5 (1993)-305, 584 (hereinafter referred to as "ankle-compliance control") to the two-leg-compliance control.
If the two kinds of control are used, it will be possible to rotate the 1st ankle in the direction in which the unexpected moment of the 1st foot floor reaction force is canceled, as illustrated in FIG. 44, such that the 1st foot heel lands on the floor. Accordingly, the robot will not turn over when the phase shifts to the succeeding one-leg supporting period.
However, if the two-leg-compliance control and the ankle-compliance control are simply combined to be used, the two kinds of control interferes with each other and causes either or both of the total floor reaction force and the foot floor reaction force to deviate from desirable values or control oscillates.
An object of the present invention is to solve the drawbacks and to provide a control system for a legged mobile robot which can ensure to control the actual floor reaction force acting on the robot easily and appropriately, while preventing the problem of interference from occurring.
Further, if the robot walks on the floor whose level is different than expected, i.e. whose level is lower than expected, the acceleration of the body may become excessive, resulting in the increase in the foot-landing impact. In addition, the robot has the natural vibration determined by its own mechanism and mass, which causes the robot to displace in the vertical direction. The magnitude of the displacement caused by the natural vibration is slight, but it may sometime degrade the contactability of the robot foot with the floor. It is therefore preferable to conduct a compliance control on the force components of the actual floor reaction force acting on the robot.
A second object of the present invention is to provide a control system for a legged mobile robot which can ensure to absorb the foot-landing impact and enhance the contactability of the robot foot with the floor by conducting a compliance control on the force components of the actual floor reaction force acting on the robot.
A third object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot appropriately, even when walking on the floor having not only a slant or undulation extending over a relatively long distance, but also on an unexpected local slant or level difference, without being affected thereby.
A fourth object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot appropriately such that a posture stabilization control of a legged mobile robot is facilitated.
A fifth object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot appropriately such that the contactability of robot foot with the floor is enhanced so as to prevent the slippage and the aforesaid spinning that can occur during walking from happening.
Further, by controlling the force component of the actual foot floor reaction force acting on each robot foot independently, for example, by increasing the force to propel the robot in the advance direction for the foot whose floor-contact pressure is high, while decreasing that for the foot whose floor-contact pressure is low, it becomes possible to enhance the posture stability of the robot such that the slippage can effectively be suppressed. Furthermore, by controlling the force component of the actual foot floor reaction force acting on each robot foot appropriately, it becomes possible to prevent one foot being a load to the force to be generated by the other foot to propel the robot in the advancing direction and some similar problems, thereby enabling to decrease the load to be exerted on joint actuators.
A sixth object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the force component of the actual foot floor reaction force acting on each robot foot independently such that the posture stability of the robot is enhanced and the load to be exerted on the joint actuators is decreased.
A seventh object of the present invention is to provide a control system for a legged mobile robot which can ensure to control the floor reaction force acting on the robot appropriately such that the load to be exerted on actuators of the robot is decreased.